Spark-gap circuits



July 26, 1960 R. A. FITCH 2,946,923

SPARK-GAP cmcuns Filed June 20. 1958 2/ 51 I SW! 4 F|G.l.

T1 c22 j :2? w L11 -u2 INVENTOR RICHARD ANTHONY FITCH ATTORNEYS United States Pal fit Claimspi'iority, application GreatBritain June 20, 19 57 {5 Claims c1. s srs) This invention relates to spark-gap circuits and hasparticular application where it is desiredtotrigger a number of spark gaps simultaneously.

Such an application arises, for example, in devices for bringing about thermonuclear reactions in an ionlsed gas,

such devices comprising a discharge tube containing. the

gas through which a large electric current is passed. One

method of producing such a current is to discharge a number of parallel'-connected condensers through the gas in the tube; It isdesirable that the rate of rise of current through the tube should be as large as possible, and it is therefore necessary to reduce the inductance of the discharge circuit to a minimum.

" 'A form of switch suitable for connecting the condensers to the discharge tube is a triggered spark-gap, but if all the condensers are connected through one large gap, all the discharge currents from the condensers flow through this gap, the inductance of which is relatively large. It would thereforebe preferable todischarge each condenser (or perhaps each relatively small group of condensers) through. a smaller spark gap,when the in-, a

and the, total inductance correspondingly reduced.

A fu'ither advantage of discharging each bank condenser through its own spark gap is that if one condenser 'ductances of the several smaller gaps would be in parallel fails by reason of an internal short circuit, the other condensers cannot discharge through it (unless thelfault' develops at the instant the bank .is discharged, and eveni'then the amount of energy' taken'by thecondenser inparallel with a low impedance load is small) but discharge slowly through the charging resistor of the faulty capacitor. The discharge of a large condenser bank through such a faulty condenser may cause a dangerous explosion.

However, a difliculty which arises in employing an arrangement of this kind is to ensure that all the smaller spark gaps are triggered within a time interval short enough to ensure therapid rise of current already referred to. This requires that the breakdown of one gap before the others should not, by cancelling the voltage across the other gaps, prevent them breaking down also.

In practice it is not possible to ensure that all the gaps will break down quite simultaneously, and it is one object of the present invention to provide a spark gap circuit in which although all the gaps may not break downsimultaneo'usly, the aforesaid cancelling efiect is eliminated so that all the gaps will in fact break down.

According to the present invention, in a spark-gap circuit comprising a plurality of electrical energy sources each connected through a connection and spark-gap means to a common load, said spark-gap means comprises a separate spark gap connected between each source and its connection to the load, said separate gaps are arranged to be triggered substantially simultaneously, the connections are of the distributed type and of equal electrical length, and the length of each said con- .nection is sufficient to delay the arrival of the source voltage at the load for a time greater than half the maximum difference in the breakdown times of the several gaps. The sources may be condensers and the distributed connections may be coaxial cables.

Each gap may be triggered substantially simultaneously by a voltage pulse produced by discharging a further energy source, which may be a condenser, through a common master spark-gap into a 'further distributed connection to said gap. g

Said gaps may each comprise two main electrodes and an irradiating electrode, said main electrodes being con- .nected respectively to the source and through the distributed-type connection to the load, and. said triggering pulse being applied to said irradiating electrode ,To enable the natureof the present invention to be more readily understood, attention is directed towards the accompanying drawingswherein:

Fig. 1 is a circuit diagram of a condenser bank discharging circuit including byway of example, a sparkgap circuit according to the present invention.

Fig. 2 is a cross section of a spark-gap electrode structure.

Referring firstly to Fig. l, condensers C11 and C12 represent for simplicity two of a bank which'it isdesired todischarge in parallel through a load which in the present embodiment is gas discharge tube 5 having upper and lower electrodes 6 and 7. The condensers have separate charging resistors R11 and R12 connectedrto a positive voltage source. Each condenser is connected to the tube 5 through a separate spark gap and coaxialcable, C1 1 being connected through the gap SG11 and the cable D11, and C12 through the gap S612 and the cable D12.

,A cross-sectional view of one of these spank gaps is shown in Fig. 2. It consists of two circular main electrodes land 2 havingajmain gap 4 between them, the electrode 2 having a central hole into which projects an irradiating electrode 3 which does not quite extend to the .surface of the latter electrode, where the bore. of the hole constricts. The electrodes may operate in air or be sealed in an evacuated or gas-filled tube. The condensers C11 and C12 are connected tothe electrodes 1 of their respective spark gaps, the electrodes 2 of which are connected to the electrode 6 ofthe discharge tube. The irradiating electrodes .3 of SG11 and 8612 are connected through equal-length coaxial cables D1 and D2,-

and condensers C21 and C22 respectively, the latter being of relatively low capacity, to the electrode 2 of a master spark gap SGl. The primary winding of a step-up transformer T1 is connected through a switch SW1 across a condenser C1 which is charged through a resistor R1 from a positive voltage source. The electrode 3 of the master spark gap 861 is connected to the secondary winding. Inductances L11 and L12 are connected between the sides of C21 and C22 remote from $61 and earth.

Assuming C11 and C12 to be charged to a positive potential insuflicient to break down SG11 and S612, and C1 to be also charged, the circuit is triggered by closing SW1. This results in a positive voltage pulse of relatively slow rise-time being generated in the secondary winding of T1, which charges C21 and C22 through L11 and L12, the charging current producing a preliminary small discharge between electrodes 2 and 3 of $61 as hereinafter explained. When the voltage across C21 and C22 reaches a sufficiently high voltage, the master gap SGl breaks down and effectively shorts one side of C21 and C22 to earth. As a result very fast negative pulse travels down each of the cables D1 and D2 to the electrodes 3 of SG11 and S612, which, since the electrodes 1 are at a positive potential, provides sufficient overvoltage to break down the gap. Rapid initiation of the breakdown is ensured by the configuration of the irradiating .SGl breaks down.

'6 and will-tend-to travel on up D12-'towards-'SG 12. It 'SGlZ has not broken down by the timethe voltage front reaches it, it willnot breakdown at-all,- since-there is now no voltage between its electrodes 1 and 2. Thus to ensure that SG12 does break down, the time taken for the front to travel along D11 and D12 is made greater than the'time interval within which the two gaps-break down, i.e. D11 and D12 are each of such a length as to delay the voltage front for a time greater than half the maximum 'diiference-in the breakdown time of'the gaps. For example, in'oneembodiment employing 144 1 t. condensers "and 1-44 sparkgaps,the uncertainty in the breakdown time of'the gaps was '10 m secs. and the coaxial cables joining each gap to the discharge tube "were thus each made long enough to delay the voltage front by more than 5 mg secs. The condensers C21 etc.

were each of 250 pf. in this embodiment.

The inductances L11 and L12 are chosen so as to present a low impedance to the charging pulse generated in L1, but a high impedance to the fast'pulse produced when has a very short rise time. It will be appreciated that the length of the time interval within which SGll, S612 and the other condensers in the bank are triggered depends largely on this rise time, owing to the variation in breakdown characteristics among the spark gaps. D1 and D11, and D2 and D12 have the same characteristic impedances so that the triggering pulses travel on down D1 and D2 and are not reflected back atthe gaps.

Circuits according to the present invention may use spark gaps other than the type described with reference to Fig. 2, and mayemploy other forms of triggering circuit than the one described. -Where a very large number of gaps is to be triggered, the single master gap SG1 may be replaced by a plurality of master gaps each triggering a group of gaps, the master gaps being triggered in turn by a master master gap.

The distributed connections need not be coaxial, but

SG1, which carries the relatively small discharge currents of C21 and C22, is designed to have a very low inductance so that the over-voltage pulse ascapze 4 may for example be of the parallel-strip transmission-line type.

In this specification, a condenser is to be taken as including two or more condensers connected in series or parallel.

I claim: a

1. A spark-gap circuit comprising a plurality of storage condensers, a load, a -spark-gap associated with each storage condenser, spark-gap having a triggering electrode and twomain electrodes, a connection between one main electrode-of eacli'spark-gap and one sideof it'sassociated storage condenser, a distributed connection between the other aine e trqde f ea h, ark-s p and the load, and meansfor triggering the spark-gaps substantially simultaneously; the distributedconnections' being of equal length suflicientto delay the arrival of the voltage fronts from said storage condensers at the load for a time greater than half the maximum difference in the breakdown times 20 of the associated spark-gaps when triggered.

' circuit a Qlaimed in-claim 1. wherein thedistributed connections are'coaxial' cables.

3. A circuit as claimed in claim" 1 whereinsaidtriggering meanscomprisesa furthercondenser associated. with each spark-gap, lequal -l'ength distributed connectionsfbetween one. side'of each further condenser and' 'thetriggering electrodelof its associated sparkfgap', a master.'sparkgap having twomainr elect'rode's', a connection: between the other side-of each further condenser and one niain electrode of theirriaster spark-gap, a connection having a high impedance. fo'rjfas't pulses connected b'etweenjsaid one side of each further condenser and the other main electrode vofthe master spark-gap,'and means for charging said further condensers to the break-downvoltage of ,master. spark-gap.

4. A a ines claimedin claim 1 wherein the triggering electrode of each associated spark-gap is an irradivating electrode.

5. Acircuit as claimed in claim 3 wherein said maste r spark gap-comprises an irradiatingelectrode and said chargingz means is connected tosaid irradiating. electrode.

" References @ited in th e file of this patent UNITED STATES PATENTS 2,478,901 Edgerton Aug, 1 6, 1949 2,478,907 Edgerton Aug. 16, 1949 2,722,629 Germeshausen Nov, 1, 1955 

